This invention relates to a copying lens system for use in a copier, etc. More particularly, this invention relates to a copying lens system that is capable of reduced, size-for-size and enlarged copying at ratios of from about 0.3X to 2.0X, that has brightness in the F-number range of from about 1:8 to 1:6:7 and that covers half-view angles (.omega.) of from about 20 to 27 degrees.
With the recent demand for reducing the size and cost of copiers, a need has arisen for using smaller and less expensive copying lenses. Copying lens systems that have heretofore been proposed for meeting the need for reduction in size and cost include a four-unit-four-element symmetrical type that has an F number of about 1:5.6 as described in Japanese Patent Publication No. 51091/1985, as well as a three-unit-three-element asymmetric type that has an F number of about 1:8 as described in Unexamined Published Japanese Patent Application No. 180924/1988. However, the four-unit-four-element symmetrical type is still unsatisfactory in terms of size and cost. On the other hand, the lens system described in Unexamined Published Japanese Patent Application No. 180924/1988 which is of a three-unit-three-element composition consists of the necessary minimum number of lens elements and yet is capable of producing a flat image However, the lens elements used have such high refractive indices that the overall lens system is not completely satisfactory in terms of cost. Speaking of its performance, this lens system has an angle coverage (2.omega.) of about 40 degrees and this should be expanded a little further without compromising the compactness of the system. In addition, currently available copying lens systems typically have a zooming range of 0.6X-1.4X and are incapable of meeting the need of consumers today for using their copiers in an expanded zooming range of from 0.5X to 2.0X and even from about 0.3X to 2.0X.